Photonic integrated circuits (PICs) provide an integrated technology platform increasingly used to form complex optical circuits. This technology allows many optical devices to be integrated on a single substrate. For example, PICs may comprise integrated lasers, integrated receivers, waveguides, detectors, semiconductor optical amplifiers (SOAs), and other active and passive optical devices arranged in various configurations.
Asymmetric twin waveguide (ATG) technology has proven to be a promising method for optoelectronic integration by virtue of the need for only a single epitaxial growth step and a simple fabrication process for even the most complex PIC design. The ATG design significantly reduces modal interference by substantially confining different modes of light to propagation in different waveguides. This is accomplished by designing the waveguides such that the mode of light that propagates in a waveguide has a different effective index of refraction than the mode of light that propagates in the adjacent waveguide. This feature isolates the light propagating in each waveguide, which lends itself to the specialization of functions performed by the waveguides. Light may be transferred between the waveguides via lithographically defined lateral taper couplers, resulting in high-performance lasers, p-i-n and avalanche photodiodes, SOAs, and integrated combinations of these fundamental photonic functionalities. U.S. Pat. Nos. 6,381,380, 6,330,387, 6,483,863, and 6,795,622, the contents of which are hereby incorporated herein by reference in their entirety, provide a description of ATG and various embodiments of ATG.
Applicants have noted unwanted absorption of light in the active layers comprised in couplers of ATG light emitting devices such as lasers and semiconductor optical amplifiers. Electrical pumping has been used to compensate for such absorption loss. However, pumping the taper is undesirable and difficult in some applications. For example, in an integrated electro-absorption modulated laser, a p-n junction is formed at the passive waveguide and must be biased independently from the active junction. Thus, the current through the taper must return to the n-contact via the narrow n-type path between the active and passive waveguides. This narrow and resistive current path makes pumping the taper difficult.